1. Technical Field
The invention is related to time division multiplex (TDM) video cassette recorders (VCR's) and tape players wherein video timing and synchronization signals must be reconstructed or recovered over noise and other distortion.
TDM recording techniques in which the present invention finds use are not necessarily restricted to VCR's but are useful in magnetic disk recording and optical disk recording, for example. Various TDM VCR's are disclosed in U.S. Pat. No. 4,300,161 and in U.S. patent application Ser. No. 725,873, filed on Apr. 22, 1985, entitled "Video Reproducing Apparatus" by Carl N. Schauffele.
2. Description of the Related Art
Superimposing pilot pulses on a video signal for various reasons is well-known in the art. For example, U.S. Pat. Nos. 4,360,824 and 3,962,725 teach recording a pilot signal on its own separate track on the video tape. U.S. Pat. No. 4,115,800, teaches recording a pilot signal having a frequency less than the horizontal line frequency f.sub.H. U.S. Pat. No. 4,084,176, teaches recording a pilot pulse at a frequency greater than the FM carrier frequency f.sub.c, and phase locking the pilot pulse to the colorburst during playback. A disadvantage of this latter teaching is that a pilot signal recorded at a frequency higher than f.sub.c is subject to well-known high frequency losses characteristic of the magnetic tape media.
3. Description of the Problems of the Related Art
Unreliable Sync Pulses PA1 Skew Error
Loss of timing or synchronization pulses is a well-known problem in video cassette recorder (VCR) technology. This problem is particularly acute in VCR systems employing time division multiplexing (TDM) in which the luminance and chrominance signals are compressed in time prior to being recorded. Typically, the timing circuits of a VCR rely on video horizontal synchronization pulses (H.sub.sync) to produce and synchronize timing pulses used, for example, to control time division multiplexing.during recording and playback in the well-known manner. The problem with this technique is that the H.sub.sync pulses are not reliable. For example, during playback, drop-outs in the video signal may cause an absence of one or more of a succession of H.sub.sync pulses, so that timing synchronization between the VCR timing circuits and the recorded video signal may be lost for one or more horizontal lines in a given video frame. Furthermore, the leading edge of the H.sub.sync pulses may be corrupted or masked by white noise or FM burst noise, or may even be mimicked by impulse noise spikes.
An effective change in tape length between recording and playback (due to a change in tape tension) causes a timing ("skew") error to accumulate (grow) steadily during recording across one complete tape track. This skew error is therefore different at the two ends of each track but is repetitive from track to track. (In fact, this repetitive pattern changes very slowly, due to the inertia of the head and drum assembly.) The accumulated skew error causes an abrupt timing jump at one end of each track just when the heads are switched and recording begins at the other end of the next track. Such errors may cause a horizontal shift or "flag waving" at the top of the video image. Typically, the solution to this problem is to provide a mechanical adjustment to the tape tension, a particularly awkward nuisance to the consumer.
Skew errors distort the horizontal sync pulse of the video signal. A TDM VCR must rely on the sync pulses for timing and control. The effect of skew error is greatly exacerbated in a TDM VCR. TDM circuits typically include digital storage devices which require consistent timing pulses of extremely fine accuracy to avoid catastrophic distortions of the video signal. For example, the chrominance signal is time compressed in a TDM VCR by a factor of about 6 (typically), so that any deviation in timing is increased six-fold.